Microcavity optical resonators are a basic element of many photonic circuits, including filters, sensors, lasers and the like. Ever-increasing demands for improvement in the operation of microcavity resonators are associated with parameters such as their Q-factor, tunability (filtering), robustness and efficiency. Particularly important realizations of the optical microcavity are the “microsphere” and “microtoroid”. An optical microsphere is used to form an optical whispering-gallery-mode (WGM) resonator that supports a special set of resonator modes. These resonator modes represent optical fields confined to an interior region of the microsphere, propagating around the “equator” of the sphere in association with the total internal reflection at the boundary of the sphere. Microspheres with diameters on the order of 10-100 microns have been used to form compact optical resonators. Since these resonators exhibit dimensions much larger than the wavelength of an associated optical signal, the optical loss associated with the finite curvature of the resonator is generally very small. The primary sources of optical loss include optical absorption in the material of the sphere and optical scattering due to the homogeneity of the sphere (e.g., irregularities on the surface of the sphere), both of which can be controlled by the fabrication process. As a result, a relatively high Q-factor can be achieved with an optical microsphere, allowing for the microsphere to find use as a highly accurate optical sensor or laser.
However, in applications where the microsphere is used as an optical sensor, the material being “sensed” needs to be placed in close proximity to the sphere. In some conventional arrangements, the material is contained with an optical microfiber probe which must be positioned at the surface of the microsphere, which is both awkward and difficult to maintain and control on a repeated basis.
Additionally, optical microspheres are difficult to use as a filtering element, since they are relatively rigid in structure and difficult to “tune” to adjust the wavelength(s) passed/blocked by the microsphere device. Another problem, which is characteristic for the microsphere optical cavity, is the very dense and practically chaotic distribution of its resonant frequencies, making the microsphere difficult to use in filtering applications.
Recently, an optical microcapillary has been demonstrated as a microfluidic optical sensor that overcomes some of the above-mentioned microsphere problems. An exemplary prior art liquid-core optical ring resonator sensor (LCORRS) 1 is shown in FIG. 1. LCORRS 1 comprises a silica capillary 2 coupled to a waist area 3 of an optical fiber taper 4. An optical signal is coupled into optical fiber taper 4 and propagates therealong (the direction of the propagating optical shown by the arrows in FIG. 1). As the propagating signal reaches waist area 3, a portion of the signal will be evanescently coupled into capillary 2 and begin to circulate as whispering gallery modes (WGMs) inside the capillary wall. This coupling is the result of the relatively thin dimension (on the order of a few microns) of the capillary wall.
In use as a sensor, a liquid or gas being tested is introduced to optical microcapillary 2, as shown in FIG. 1, where the WGMs of the probing optical signal will interact with the sample and, as a result, modify the optical signal propagating along optical fiber taper 4. An analysis of the optical signal exiting optical fiber taper 4 can then be used to define the characteristics of the sample being tested. An LCORRS has also been demonstrated as a laser, by utilizing an active optical fluid passing through microcapillary 2.
The ability to pass the sensing/lasing material through a microcapillary and provide optical sensing/lasing with an adjacent microfiber taper results in an optical sensor that is more convenient to use than the above-described microsphere and microtoroid. In particular, the sample liquid is situated inside the capillary and the liquid flow does not perturb the coupling between the microfiber and the capillary. Moreover, the capillary-microfiber assembly can be “fixed” within a low-index polymer matrix, forming a device that is extremely robust and convenient for many applications.
However, problems remain with the sensitivity that may be achieved with an LCORRS device. That is, since the microcapillary is not “bounded”, the circulating WGMs will tend to spread outwards along the length of the microcapillary (shown by the arrows within the microcapillary 2 of LCORRS 1). Thus, the localized eigenmodes of the circulating WGMs do not remain confined to waist region W and, therefore, the Q-factor of the device is rather limited. In other words, the light launched into the LCORRS device cannot dwell is the waist area for a long time and escapes in both directions along the capillary axis. As discussed above, an important feature of “sensing” optical signals is creating and maintaining a “high Q”, allowing for the sensing process to exhibit extremely high levels of sensitivity.
Thus, a tension remains between these two types of prior art sensors, where the microsphere/microtoroid has the advantage of desirable high Q-factor, but is not robust or practical in implementation, while the LCORRS sensor is very robust and applicable in many situations, but has limits on the Q-factor that may be achieved.
Thus, a need remains in the art for a microresonator that is capable of achieving the high Q-factor associated with the microsphere, yet is practical and robust in implementation.